1. Field of the Invention
The present invention relates to improvement of a high electron mobility field effect semiconductor device of a multi-hetero-structure type including a plurality of two-dimensional electron gas (2DEG) layers each formed of a carrier supply layer and a carrier transport layer.
Although a single layer can establish two 2DEG layers or can supply carriers to two carrier transport (channel) layers, they can be counted as two adjoined layers, unless otherwise specified, in this specification.
Recently, high electron mobility transistors (HEMT's) have been increasingly utilized for high-frequency communications employing, for example, microwaves and higher-frequency waves. There still remains a need for further improvement of HEMT's to increase performance and efficiency thereof.
2. Description of the Related Art
In an ordinary configuration of a high-power HEMT, to develop a high-power output signal, there has been usually adopted a multi-hetero-structure including a plurality of layer stacks or multi-layered blocks each having a carrier supply layer and channel layer.
FIG. 11 is a cross-sectional side view of a primary portion of a conventional example of an HEMT having a double hetero-structure.
The configuration of FIG. 11 includes a semi-insulating GaAs substrate 1, a non-doped GaAs buffer layer 2, an n-type AlGaAs carrier supply layer 3, a non-doped GaAs carrier transport layer 4, an n-type AlGaAs carrier supply layer 5, an n-type GaAs cap layer 6, a source electrode 7, a drain electrode 8, a gate electrode 9, and 2DEG layers 10 and 11.
Since n-type AlGaAs carrier supply layers have an energy gap wider than that of a non-doped GaAs carrier transport layer, a narrow potential well is formed in the neighborhood of each interface of the carrier transport layer. Carriers existing in the supply layer fall into a potential well to generate a two-dimensional electron gas layer.
As compared with a single hetero-structure HEMT, the double hetero-structure HEMT develops a higher value of a common gate drain-source current I.sub.dss per unitary gate width, namely, a higher output current.
In this connection, regardless of a conduction type, namely, whether carriers are electrons or holes, the abbreviated terms HEMT and 2DEG are used herebelow in this specification. Namely, when the carriers are holes, the abbreviation E (electron) designates holes.
In an HEMT having a double hetero-structure, according to the gate control theory, the two-dimensional carrier gas layer 10 of the lower-hetero-interface is controlled after the upper hetero-interface or hetero-boundary is completely depleted. The distance between the gate electrode 9 and the lower hetero-interface is naturally larger than that between the gate electrode 9 and the upper hetero-interface. Thickness of the depletion layer is consequently increased and hence the gate capacitance becomes small.
In general, the transconductance or mutual conductance g.sub.m is an essential factor for transistors. This is also the case with the HEMT's. The transconductance g.sub.m is proportional to the gate capacitance C.sub.gs.
In the double hetero-structure HEMT, since the distance between the upper hetero-interface and the gate electrode 9 is different from that between the lower hetero-interface and the gate electrode 9, the transconductance g.sub.m of the upper hetero-interface is larger than that of the lower hetero-interface.
FIG. 12 is a graph showing relationships between a gate bias voltage V.sub.gs and the transconductance g.sub.m of the HEMT of FIG. 11. The abcissa and the ordinate respectively stand for the gate bias voltage V.sub.gs and the transconductance g.sub.m.
As can be seen from this graph, in the characteristic curve representing the relationships between the gate bias voltage V.sub.gs and the transconductance g.sub.m of the HEMT, there take place two peaks having different heights. The high and low peaks are associated with the upper and lower hetero-interfaces or hetero-boundary regions, respectively.
For a high-power HEMT to develop a favorable linearity, it is desirable to have a flat characteristic line of the relationships between the gate-bias voltage V.sub.gs and the transconductance g.sub.m. In the HEMT of FIG. 11, however, there appear two peaks having different heights in the transconductance g.sub.m, leading to a poor linearity.